Resistance spot welding, which is a type of lap resistance welding, is typically used to join overlapping steel sheets. This welding method is a method to join two or more overlapping steel sheets by applying a high welding current for a short time between a pair of electrodes squeezing the steel sheets from above and below. A point-like weld is obtained using the resistance heat generated by passing the high welding current. Such a point-like weld is referred to as a nugget and is the portion were both of the overlapping steel sheets fuse and coagulate at a location of contact between the steel sheets when current is applied to the steel sheets. The steel sheets are joined in a point-like manner by this nugget.
In order to obtain good weld quality, it is important to form a nugget which has an appropriate diameter. The nugget diameter is determined by welding conditions such as the welding current, welding time, electrode shape, electrode force, and the like. Therefore, to form an appropriate nugget diameter, the above welding conditions need to be set appropriately in accordance with the conditions of parts to be welded, such as the material properties, sheet thickness, number of sheets overlapped, and the like.
For example, when manufacturing automobiles, spot welding is performed at several thousand points per automobile, and workpieces that arrive one after another need to be welded. At this time, if the conditions of parts to be welded, such as the material properties, sheet thickness, number of sheets overlapped, and the like are identical, then at each welding location, the same nugget diameter can be obtained under the same welding conditions such as the welding current, welding time, electrode force, and the like. During consecutive welding, however, the surfaces of the electrodes in contact with the parts to be welded gradually wear as welding is performed multiple times, so that the contact area between the electrodes and the parts to be welded gradually expands. If the same welding current as during the first welding is applied after the contact area has thus expanded, the current density in the parts to be welded lowers, and the temperature rise in the weld is reduced. The nugget diameter therefore decreases. Hence, for every several hundred to several thousand spots of welding, the electrodes are either dressed or replaced, so that the electrode tip diameter does not expand excessively.
A resistance welding device provided with a function (stepper function) to increase the welding current after welding a predetermined number of times, so as to compensate for the reduction in current density due to wear of the electrodes, has also been used conventionally. To use that stepper function, the above-described pattern for changing the welding current needs to be set appropriately in advance. Doing so, however, requires that tests or the like be performed to derive a pattern for changing the welding current that corresponds to numerous conditions of parts to be welded and welding conditions, which is highly time-consuming and expensive. The state of progress of electrode wear also varies during actual work. Therefore, the predetermined pattern for changing the welding current cannot always be considered appropriate.
Furthermore, if there is a disturbance at the time of welding, such as when a point that has already been welded (existing weld) is located near the point being welded, or when the surface of the parts to be welded is highly uneven and a contact point between the parts to be welded is located near the point being welded, then current diverts to the existing weld or the contact point. In such a state, the current density is reduced at the position to be welded directly below the electrodes, even when welding under predetermined conditions. A nugget of sufficient diameter therefore cannot be obtained. In order to compensate for this insufficient amount of heat generated and to obtain a nugget of sufficient diameter, it becomes necessary to set a high welding current in advance.
Techniques such as the following have been proposed to resolve the above problem. For example, JP H9-216071 A (PTL 1) discloses a control unit of a resistance welder that obtains a set nugget by comparing an estimated temperature distribution of the weld with a target nugget and controlling output of the welder.
JP H10-941883 A (PTL 2) discloses a method of controlling welding conditions of a resistance welder to achieve good welding by detecting the welding current and the voltage between tips, performing a simulation of the weld by heat transfer calculation, and estimating the formation state of the nugget.
Furthermore, JP H11-33743 A (PTL 3) discloses a welding system that first uses the sheet thickness of the parts to be welded and the welding time to calculate the cumulative amount of heat generated per unit volume that allows good welding of the parts being welded and then adjusts the welding current or voltage that yields the calculated amount of heat generated per unit volume and unit time. A good weld can be achieved using this system, regardless of the type of parts to be welded or the wear state of the electrodes.
With the resistance spot welding methods in PTL 1 and PTL 2, however, complicated calculations are necessary in order to estimate the temperature of the nugget based on a heat transfer model (heat transfer simulation) or the like. The structure of the welding control unit not only becomes complicated, but the welding control unit itself also becomes expensive.
The resistance spot welding method recited in PTL 3 always allows good welding regardless of the degree of electrode wear by using the cumulative amount of heat generated as a target value and controlling the welding current or voltage. If the set conditions of parts to be welded and the actual conditions of parts to be welded greatly differ, however, for example in cases such as when there is a disturbance nearby such as the aforementioned existing weld, when the time variation pattern of the amount of heat generated changes greatly in a short period of time, or when welding hot-dip galvanized steel sheets with a large coating weight, then adaptive control cannot be performed accurately with this welding method. Accordingly, even if the final cumulative amount of heat generated can be matched to the target value, the form of heat generation, i.e. the pattern of the amount of heat (change in temperature over time) in the weld, deviates from the pattern of the amount of heat that yields the desired good weld. In this case, the necessary nugget diameter might not be obtained, or splashing may occur. For example, when the effect of shunt current is large, then attempting to match the cumulative amount of heat generated to the target value causes significant heat generation near a location between the electrode and the steel sheet instead of between the steel sheets and increases the likelihood of splashing from the steel sheet surface.
Furthermore, all of the techniques in PTL 1 to PTL 3 effectively address the change when the electrode tip wears but do not at all take into account the case of shunt current having a large effect, such as when the distance from an existing weld is short. Hence, adaptive control sometimes does not actually work.